metamorphic
River Cobble (likely Serpentinite or Greenstone)
Serpentinite / Greenschist facies rock
Hardness: 3-6 (variable); Color: Mottled green, grey, and brownish-red; Luster: Waxy to dull when wet; Structure: Fine-grained, microcrystalline, often with veining; Specific Gravity: 2.2-2.9
- Hardness
- 3-6 (variable)
- Color
- Mottled green, grey, and brownish-red
- Luster
- Waxy to dull when wet
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Physical properties
Hardness: 3-6 (variable); Color: Mottled green, grey, and brownish-red; Luster: Waxy to dull when wet; Structure: Fine-grained, microcrystalline, often with veining; Specific Gravity: 2.2-2.9
Formation & geological history
Formed through the hydration and metamorphic transformation of ultramafic rock from the Earth's mantle, typically in subduction zones. This specimen shows signs of fluvial erosion (rounded by river action).
Uses & applications
Used for architectural stone, decorative carvings, and as a source of asbestos in some industrial contexts. Tumbled versions are popular for aquarium decor or landscaping.
Geological facts
Serpentinite is the official state rock of California. It often forms the base of unique ecosystems because its soil chemistry is toxic to many common plants but supports rare endemic species.
Field identification & locations
Identify by its characteristic waxy 'serpent-like' green color and smooth texture. Often found in riverbeds near tectonic plate boundaries or ophiolite complexes. Look for a 'soapy' feel on dry surfaces.
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